Recently, in order to improve the exhaust gas cleaning capability and mileage, a throttle valve control apparatus of the electronically controlled actuator type is being developed in which the mechanical connection between the throttle valve and the accelerator pedal is removed and the throttle valve is controlled to open and close by utilizing a signal representative of an amount of accelerator depression converted into an electrical quantity (accelerator depression amount signal) and a signal representative of engine operating conditions or vehicle running conditions (for example, an engine rpm signal, a gear position signal).
The opening and closing of the throttle valve is achieved by a drive motor which is actuated by a command from a vehicle control apparatus comprising an operation control unit in which an optimum opening degree is successively calculated in response to a signal representative of an engine operation condition or a vehicle travelling condition. Therefore, if an electronically controlled actuator including the above-mentioned drive motor is disabled while the vehicle is moving, the throttle is held open, which may result in the vehicle running out of control and an accident. Therefore, it has been necessary to provide a safety device in which uncontrolled vehicle running can be prevented even when the electronically controlled actuator becomes disabled. Examples in which such a safety device is used are disclosed in Japanese Patent Publication No. 58-25853 and Japanese Patent Laid-Open No. 61-215436. More particularly, in the former, an electromagnetic clutch is provided for disconnecting a throttle valve shaft from the drive motor when the electronically controlled actuator becomes uncontrollable and a return spring is mounted on the throttle valve shaft for completely closing the throttle valve when the electromagnetic clutch is OFF. In the latter, the throttle valve shaft is mechanically rotated through a differential gear unit in accordance with the control amount of the electronically controlled actuator or the amount of accelerator pedal depression.
However, in such a conventional throttle valve control apparatus, while uncontrolled moving of the vehicle can be prevented, the vehicle cannot be driven, posing the problem that the vehicle cannot be moved to a desired position suitable for repair. Also, when only a single return spring is used, the throttle valve cannot be completely closed if this return spring breaks, posing another danger of the vehicle running out of control, and further a strong drive motor having sufficient drive power for overcoming the return spring is necessary when a single powerful spring is used, making the control apparatus larger as the size of the drive motor is increased.
Accordingly, the present invention has been made in order to eliminate such problems of the conventional apparatus and has as its object the provision of a throttle valve control apparatus that is very safe, highly responsive and can be small-sized.